1. Field of the Invention
The present invention relates to an optical modulator having an optical interference unit which branches inputted light into two optical waveguides and combines the respective branched lights propagating through the respective optical waveguides, an optical transmitter including the optical modulator, and an optical modulating method for modulating light inputted into an optical interferometer, and a manufacturing method of the optical modulator.
2. Description of the Related Art
As a constitution of the optical transmitter used in a WDM (Wavelength Division Multiplex) system of an intermediate/long distance transmission, the combination of a DFB laser (Distributed Feedback Laser) and the optical modulator is generally used. In the conventional WDM system, there are more than several tens of wavelength channels, and as the wavelength of the input light (continuous light) which is inputted into the optical modulator, the wavelength width is about 35 nm, for example, with the C band taken as an example. Therefore, as the optical modulator, it is desired to be able to cope with the wavelength of the input light in a wide range. This eliminates necessity to manufacture optical modulators of different structures corresponding to the respective wavelength channels of the WDM system, and therefore, great advantage can be obtained in terms of cost.
In the development for the WDM system of the next generation, studies of an optical ADM (Add/Drop Multiplex) which performs add/drop and routing in accordance with the wavelength of data, optical cross connect (OXC), optical burst switching and the like have been enthusiastically made in recent years. In this next-generation WDM system, an optical transmitter capable of widely changing the wavelength of the input light in the C-band having the wavelength bandwidth of, for example, 35 nm is required. In such an optical transmitter, a wavelength tunable laser is used as a CW (Continuous Wave) light source. In this case, as the optical modulator, an optical modulator, which can be used in the wide wavelength bandwidth capable of coping with allotment of the wide wavelength of the optical transmitter, is required.
Recently, as one of the optical modulators realizing the operation in the aforementioned wide wavelength bandwidth, a Mach-Zehnder (MZ) type of optical modulator constituted of a semiconductor material is studied. FIG. 32 is a schematic diagram of a semiconductor Mach-Zehnder type optical modulator.
A Mach-Zehnder type optical interferometer 10 is constructed on a high-resistance InP substrate 1, and an optical waveguide is formed in this Mach-Zehnder type optical interferometer 10. This optical waveguide is constituted so as to branch the continuous light inputted into one end (input end) of the Mach-Zehnder type optical interferometer 10 into two and propagates them in the Mach-Zehnder type optical interferometer 10, and irradiates the modulated light, which is generated by combining the branched and propagated lights again, from the other end (output end). Of the two branched optical waveguides, a pair of electrodes 11a and 11b are formed at an upper first arm 11, and a high-frequency driving power supply 61 for driving the optical modulator is connected to the upper first arm 11 via the pair of electrodes 11a and 11b. 
A direct-current power supply 62 for biasing is connected to the high-frequency driving power supply 61. Here, voltage amplitude of the high-frequency driving power supply 61 is set as Vpp, and the bias voltage of the direct-current power supply 62 is set as Vb. Meanwhile, at a lower second arm 12, a pair of electrodes 12a and 12b are formed, and a direct-current power supply 71 for phase adjustment is connected to the lower second arm 12 via a pair of electrodes 12a and 12b. Here, the phase adjusting voltage of the direct-current power supply 71 is set as Va.
FIG. 33 is a characteristic chart showing the optical output characteristics in the input voltage of the semiconductor Mach-Zehnder type optical modulator shown in FIG. 32. What is shown in FIG. 33 is a so-called extinction curve. The horizontal axis represents the input voltage V between the electrodes of the first arm and the vertical axis represents the optical output. When the input voltage V is 0, namely, the input voltage is low-level in FIG. 33, the light propagating through the first arm 11 is the light of the same phase as the light propagating through the second arm 12. Accordingly, when both of them are combined at the output end of the Mach-Zehnder type optical interferometer 10, modulated light with the same intensity as the input light is regenerated. In this manner, the on level of the modulated light is outputted.
When the input voltage V is increased from 0, the refractive index of the light, which propagates through the first arm 11, also changes. As a result, the light propagating through the first arm 11 has a phase difference with respect to the light propagating through the second arm 12. When the input voltage V reaches a magnitude of some extent, namely, at high-level input voltage, the phase difference between the light propagating through the first arm 11 and the light propagating through the second arm 12 becomes π, and when both of them are combined at the output end of the Mach-Zehnder type optical interferometer 10, they cancel each other out, which results in extinction. In this manner, the off level of the modulated light is outputted. Here, the on level and the off level of the modulated light are respectively set at 0 and π, but if they are respectively set at the values which are the results of adding or subtracting an integral multiple of 2π to or from 0 and π, the modulation is similarly possible.
FIG. 34 is a characteristic chart of the phase difference (phase difference between the arms) of the light propagating through each of the arms with respect to the input voltage of the semiconductor Mach-Zehnder type modulator shown in FIG. 32. Here, the horizontal axis represents the input voltage V between the electrodes of the first arm 11 and the vertical axis represents the phase difference of the light propagating through the first arm 11 and the light propagating through the second arm 12. In an ordinary Mach-Zehnder type optical modulator, the amplitude Vpp of the high-frequency driving power supply 61 is set so that the phase difference of the lights from the respective arms changes from 0 to π between the on level and the off level, as shown in FIG. 34. As a result, it is possible to modulate the maximum optical output intensity and the minimum optical output intensity on the extinction curve shown in FIG. 33 respectively corresponding to the on level and the off level of the light.
In the general Mach-Zehnder type optical modulator, the low level and the high level of the input voltage V respectively correspond to the on level and the off level of the optical output as shown in FIG. 33. Namely, when the input voltage V is high-level, the optical output becomes off-level, and when the input voltage V is low-level, the optical output becomes on-level. In this case, in the input voltage V, the central position of the input voltage applied at the time of the on level of the modulated light and the input voltage applied at the time of off level is set by the bias voltage Vb of the direct-current power supply 62. Accordingly, when the input voltage V is changed from 0 to Vpp, the bias voltage Vb is set at (Vpp/2).
As shown in FIG. 34, in the semiconductor Mach-Zehnder type optical modulator, the phase difference of the lights propagating through the respective arms nonlinearly increases with respect to the input voltage V, and its gradient increases as the input voltage V becomes larger. This nonlinear characteristic remarkably appears, for example, by constructing the core layer of the optical waveguide by a multiple quantum well (MQW) layer, and utilizing the electro-optical effect by the quantum-confined Stark effect. In the extinction curve in FIG. 33, the space between the mountain and the valley is shown to be gradually shorter. This results from the above-described nonlinear characteristic with respect to the input voltage V, and is the characteristic peculiar to a semiconductor. As for the operation of the semiconductor Mach-Zehnder type optical modulator in the wide wavelength bandwidth, there are conventionally several known examples.
For example, the semiconductor Mach-Zehnder type optical modulator described in the following Non-patent Document 1 is cited. FIG. 35 is a diagram showing a setting method of the drive condition with respect to a channel with short wavelength and a channel with long wavelength of the semiconductor Mach-Zehnder type optical modulator in the Non-patent Document 1. As shown in FIG. 35, in the extinction curve of the channel with short wavelength, the input voltage V before the extinction is small, while in the extinction curve of the channel with long wavelength, the input voltage V before extinction is large.
For the extinction curve having such a characteristic, in the semiconductor Mach-Zehnder type optical modulator in the Non-patent Document 1, the amplitude Vpp of the high-frequency driving power supply 61 is set by being changed in accordance with the wavelength of the input light as shown in FIG. 35. Namely, when the wavelength of the input light is short wavelength, the amplitude Vpp is set to be comparatively small (Vpp1), and when the wavelength of the input light is long wavelength, the amplitude Vpp is set to be comparatively large (Vpp2). In the Non-patent Document 1, the amplitude Vpp of the high-frequency driving power supply 61 is controlled in accordance with the wavelength of the input light, and thereby, the maximum optical output intensity and the minimum optical output intensity of the extinction curve for the wavelength of each of various input lights are modulated respectively corresponding to the on level and the off level of each of the lights.
There is also the semiconductor Mach-Zehnder type optical modulator described in the following Non-patent Document 2. FIG. 36 is a diagram showing a setting method of the drive condition with respect to the channel with short wavelength and the channel with long wavelength of the semiconductor Mach-Zehnder type optical modulator in the Non-patent Document 2. In this Non-patent Document 2, the amplitude Vpp of the high-frequency driving power supply 61 is set as the same with respect to the wavelengths of different input lights, and with the case where the wavelength of the input light is shortwave as the reference, the maximum optical output intensity and the minimum optical output intensity of the extinction curve are modulated respectively corresponding to the on level and the off level.
In the optical transmitter (optical modulator), it is desired to suppress the change of the wavelength which occurs accompanying the optical intensity modulation, namely, so-called wavelength chirping to be small (low wavelength chirping operation). This is because the intensity waveform of the light which is transmitted is lost due to wavelength dispersion of the optical fiber if the wavelength chirping is large, and as a result, the transmitting distance is limited to be short.
Especially in a optical transmitting system for medium/long distance transmission, it is required to make the total dispersion of the transmission line zero by using a dispersion compensating fiber, and to make the wavelength chirping of the optical transmitter (optical modulator) substantially equal to zero. This makes it possible to provide the system capable of keeping favorable waveform quality irrespective of the transmission distance and insusceptible to the fluctuation of the total dispersion amount of the transmission line.
In order to realize low wavelength chirping, it is general to use a semiconductor laser (for example, a laser diode LD) emitting continuous light, and an external modulator which modulates the intensity of the continuous light in accordance with the input electrical signal in combination, as an optical transmitter. Among the external modulators, a Mach-Zehnder type optical modulator of push-pull drive is considered to be promising because it can realize zero chirping in theory.
Here, FIGS. 37A and 37B are schematic diagrams showing the constitution of the semiconductor Mach-Zehnder type optical modulator of push-pull drive. FIG. 37B is a sectional view in the line XI-XI in FIG. 37A.
In the semiconductor Mach-Zehnder type optical modulator which performs a push-pull operation, the Mach-Zehnder optical interferometer is constructed by two optical waveguides (arms) 50A and 50B, and two optical couplers 51 and 52 as shown in FIG. 37A, and the continuous light incident from one end portion is temporarily branched at the optical coupler 51, and is guided to the two arms 50A and 50B, after which, the branched lights are coupled by the optical coupler 52 again, and the modulated light is emitted from the other end portion. The optical waveguides 50A and 50B have the structure in which an optical waveguide core layer 55 is sandwiched by an upper and lower conductive semiconductor clad layers 53 and 54 as shown in FIG. 37B. As shown in FIGS. 37A and 37B, electrodes 56A and 56B are respectively formed on the two arms 50A and 50B. Further, as shown in FIG. 37A, driving power supplies 57 and 58 are respectively connected to these electrodes 56A and 56B, so that electrical signals (voltage) are inputted into the respective electrodes 56A and 56B from these driving power supplies 57 and 58. These electrodes are called signal electrodes. As shown in FIG. 37B, an electrode 59 is formed on a lower side (backside) of the optical waveguides 50A and 50B via the conductive semiconductor clad layer 54. This electrode 59 is grounded and at the ground potential, and therefore, called a ground electrode.
When the electrical signals are inputted into the signal electrodes 56A and 56B from the two driving power supplies 57 and 58, voltage is applied to the respective two arms 50A and 50B, and the respective refractive indexes of the two arms 50A and 50B change in accordance with the magnitude of the applied voltage due to the electro-optical effect.
Next, the operation of the Mach-Zehnder type optical modulator utilizing the electro-optical effect constituted as above will be explained.
First, when the ON state of light is to be outputted, the voltage of V0 is equally applied to the respective electrodes 56A and 56B on the two arms 50A and 50B. In this case, the refractive indexes of the two arms 50A and 50B respectively change due to the electro-optical effect. As a result, the phases of the lights propagating through the insides of the two arms 50A and 50B after branched at the optical coupler 51 change as compared with those when the voltage is zero. However, since the equal voltage is given to the two arms 50A and 50B, the change amounts of the phases of the lights propagating through the two arms 50A and 50B are equal. Accordingly, when these lights are coupled with the optical coupler 52 again, the phases of these lights become equal, and therefore, the light with the same intensity as the original incident light is regenerated. By outputting the light which is regenerated to have the same intensity as the incident light, the ON state of the light is outputted.
Meanwhile, when the OFF state of light is to be outputted, voltage of V0±ΔV is applied to the respective electrodes 56A and 56B on the two arms 50A and 50B. In this case, the refractive indexes of the two arms 50A and 50B change due to the electro-optical effect, and the refractive index of the one arm 50A (or 50B) becomes low, while the refractive index of the other arm 50B (or 50A) becomes high. As a result, after the light is branched at the optical coupler 51, the phase of the light propagating through the one arm 50A (or 50B) delays and the phase of the light propagating through the other arm 50B (or 50A) advances, with the case where the light is brought into the ON state as the reference. Namely, when the phase in the case where the light is brought into the ON state is set at zero, and the side to which the phase advances is set as positive, and the side to which the phase delays is set as negative, the phases of the lights propagating through the insides of the two arms 50A and 50B change to the opposite signs from each other. The ΔV is determined so that the phase difference between these lights becomes π radian (180°) after the lights respectively propagate through the two arms 50A and 50B. Therefore, when these lights are coupled with the optical coupler 52 again, these two branched lights cancel each other out to be extinct. As a result, the optical intensity practically becomes zero, and the OFF state of the light is outputted.
In the Mach-Zehnder type optical modulator of push-pull type, the changes in the voltage applied to the two arms 50A and 50B are the same in amount and are opposite in sign on the occasion of the change of the state of ON to OFF (or OFF to ON) of light as described above. Accordingly, the changes in phase which the lights propagating through the two arms 50A and 50B receive on the occasion of change in the applied voltage are the same in amount and are opposite in sign. As a result, when the lights propagating through the two arms 50A and 50B are coupled with the optical coupler 52 again, the phase of the multiplexed light is always kept constant (except the part changing with the frequency of light). This indicates that the wavelength chirping expressed by the phase change of light per unit time becomes zero.
Incidentally, for example, Patent Document 1 describes that in the push-pull type Mach-Zehnder optical modulator as shown in FIGS. 37A and 37B, the wavelength chirping amount can be controlled by adjusting the length of the electrodes formed on the two arms.
Namely, when the lengths of the electrodes on the two arms are made equal, the phase changes of the lights propagating through the two arms become the same in amount and opposite in sign from each other as described above, and therefore, the wavelength chirping becomes zero chirping.
On the other hand, when the length of the electrode formed on one of the arms is made long, and the length of the electrode formed on the other arm is made short, namely, when the electrode lengths are made asymmetrical between the two arms, the phase change amounts of the lights propagating through the two arms become different. Namely, the light propagating through the arm on which the electrode of the longer electrode length is formed receives a larger phase change. In this case, the phase of the multiplexed light does not become constant when the lights propagating through the two arms are coupled with the optical coupler again, but the phase of the multiplexed light changes on the occasion of the change of the state of ON/OFF (or OFF/ON). As a result, chirping which is not zero occurs to the coupled light with the optical coupler. The wavelength chirping amount changes in accordance with asymmetry property of the electrode lengths.
As the Mach-Zehnder type optical modulator which performs a push-pull operation as in FIGS. 37A and 37B, there is a Mach-Zehnder type optical modulator having a capacitively loaded type electrode structure as shown in FIGS. 38A and 38B. The Mach-Zehnder type optical modulator having such a capacitively loaded type electrode structure (a kind of traveling-wave electrode) has been proposed as the modulator using GaAs related materials at first, and the GaAs related materials have been mainly studied.
Here, FIGS. 38A and 38B respectively show a plan view and a sectional view of the capacitively loaded type of Mach-Zehnder type optical modulator.
As shown in FIG. 38A, coplanar/slot-line type traveling-wave electrodes 123 and 124 which become high-frequency transmission lines are formed at both sides of two arms (optical waveguides) 120A and 120B. Electrodes 127 and 128 are discretely formed on the two arms 120A and 120B. These discrete electrodes 127 and 128 are respectively connected to the slot-line type traveling-wave electrodes 123 and 124. Portions provided with the discrete electrodes 127 and 128 function as microscopic phase modulators 132. The lengths of the discrete electrodes 127 and 128 are all the same. Namely, all the discrete electrodes 127 and 128 formed on the respective arms 120A and 120B have the same length, and when compared between the two arms 120A and 120B, the discrete electrodes 127 and 128 have the same length.
In the Mach-Zehnder type optical modulator having such a capacitively loaded type of electrode structure, an electrical signal for modulation is inputted from an end portion of one of the coplanar/slot-line type electrodes 123 and 124, and propagates on the slot-line type electrodes 123 and 124 while supplying voltage to the discrete electrodes 127 and 128 which are discretely formed. In this electrode structure, the loss is small when the electrical signal for modulation propagates through the traveling-wave electrodes, and therefore, a wide-range modulation operation is possible.
Here, of two coplanar/slot-line type electrodes, the electrode (signal electrode, signal side electrode) 123 connected to a signal terminal of a driving power supply (high-frequency driving power supply) 125 which supplies the modulation signal is formed to be narrower in the width of the electrode as compared with the electrode (ground electrode, ground side electrode) 124 which is connected to a ground terminal. Namely, in the capacitively loaded type of Mach-Zehnder type optical modulator, the shapes of the slot-line electrodes 123 and 124 connected to the discrete electrodes 127 and 128 formed on the two arms 120A and 12B are asymmetrical.
As shown in FIG. 38B (sectional view in the line XII to XII in FIG. 38A), each of the two optical waveguides 120A and 120B has the structure in which the optical waveguide core layer 103 is sandwiched by the conductive semiconductor layers 102 and 104 as the clad layers. The high-frequency driving power supply 125 for modulation is connected to the discrete electrodes 127 and 128 which are formed on the two arms (optical waveguides) 120A and 120B via the two slot-line electrodes 123 and 124. The two arms 120A and 120B are electrically connected in series to the high-frequency driving power supply 125 via a conductive semiconductor layer 102 below the arms 120A and 120B.
Accordingly, the modulation voltage V supplied from the high-frequency driving power supply 125 is distributed in the opposite directions in the same magnitude and applied to the respective core layers of the two arms 120A and 120B as shown in FIG. 38B.
As described above, in the capacitively loaded type Mach-Zehnder optical modulator, by applying the voltage for modulation to the two arms 120A and 120B, namely, the optical waveguide core layers 103, push-pull modulation operation is possible as in the push-pull drive type of Mach-Zehnder type optical modulator as shown in FIGS. 37A and 37B. Accordingly, a zero chirping operation is also expected in the capacitively loaded type Mach-Zehnder optical modulator.
As a control method of the wavelength chirping in the capacitively loaded type Mach-Zehnder optical modulator, there is the one described in Patent Document 2, for example.
Patent Document 2 describes that in a capacitively loaded type Mach-Zehnder optical modulator made of GaAs related materials, the lengths of the electrodes formed on the two arms (optical waveguides) are made equal to provide the symmetrical structures, and thereby, zero chirping is realized. In Patent Document 2, by connecting an extra electric capacitive component to one of the electrodes in parallel while keeping the lengths of the electrodes formed on the two arms equal, chirping which is not zero is realized. This is because the magnitude of the voltage applied to the arm (optical waveguide) becomes smaller in the electrode to which the extra electric capacitive component is connected as compared with that in the other, and thereby, a difference occurs to the magnitudes of the phase changes of the lights propagating through the two arms.
Incidentally, it is studied to constitute the capacitively loaded type Mach-Zehnder modulator by InP related materials as shown in FIGS. 38A and 38B.
For example, in Non-patent Documents 3 and 4, it is described that the clad layers made of InP, and the optical waveguide core layer constituted of MQW (multiple quantum well) of InGaAsP are formed on an InP substrate, and the capacitively loaded type Mach-Zehnder modulator is constructed.
As the characteristic of the InP related materials, it is cited that the wavelength which causes band edge absorption (absorption edge wavelength) is near 1.3 μm band or 1.55 μm band of the operating wavelength in the ordinary optical communication system. Thus, the InP related materials have the characteristic that the change in the refractive index per unit length and unit voltage due to the electro-optical effect is large as compared with the other semiconductor materials such as GaAs, and dielectric materials such as LiNbO3. Therefore, in the Mach-Zehnder type optical modulator made of InP related materials, it is possible to shorten the element length as compared with the optical modulators made of the other materials.
Since the wavelength which generates the band end absorption is near the operating wavelength as described above, in the case of using the InP related materials, there is a different point that loss due to absorption occurs to the light propagating through the optical waveguide from the case where the other materials are used. The absorption amount of light changes in accordance with the voltage which is applied to the optical waveguide.
Besides these arts, there are the arts described in, for example, Patent Documents 3 to 5 as the arts for adjusting chirping in the Mach-Zehnder type optical modulators.
[Patent Document 1] U.S. Pat. No. 5,991,471
[Patent Document 2] U.S. Patent Application Publication No. 2003/0190107
[Patent Document 3] Japanese Patent Application Laid-open No. 2003-322831
[Patent Document 4] Japanese Patent Application Laid-open No. 7-199133
[Patent Document 5] Japanese Patent Application Laid-open No. 9-61766
[Patent Document 6] Japanese Patent Application Laid-open No. 2004-53830
[Non-patent Document 1] “INTEGRATED TUNABLE TRANSMITTERS FOR WDM NETWORK” in Session Th.1.2.1 of International Conference, European Conference on Optical Communication 2003 (ECOC'03)
[Non-patent Document 2] “InP/GaInAsP π-shifted Mach-Zehnder modulator for wavelength independent (1530-1560 nm) propagation performance at 10 Gb/s over standard dispersive fiber” of Scientific prepublication paper journal, Electronics Letters vol. 33, 697 page, (1997)
[Non-patent Document 3] S. Akiyama et al. “40 Gb/s InP-based Mach-Zehnder Modulator with a driving voltage of 3 VPP” 16th International Conference on Indium Phosphide Related Materials, ThA1-4, 2004
[Non-patent Document 4] D. Hoffmann et al. “45 GHZ BANDWITH TRAVELLING WAVE ELECTRODE MACH-ZEHNDER MODULATOR WITH INTEGRATED SPOT SIZE CONVERTER” 16th International Conference on Indium Phosphide Related Materials, ThA1-5, 2004
[Non-patent Document 5] “Wavelength Tunable DFB Laser Array for WDM Application” in the session 3.3.1 of International Conference European Conference on Optical Communication 2002 (ECOC'02)
[Non-patent Document 6] “10 Gbit/s, 1.56 μm MULTIQUANTUM WELL InP/InGaAsP MACH-ZEHNDER OPTICAL MODULATOR” in pages 471 to 472 of Journal of technical papers, ELECTRONICS LETTERS, 4 Mar. 1993, Vol. 29, No. 5
[Non-patent Document 7] “A 40-Gbit/s InP-based n-i-n Mach-Zehnder modulator with a π-voltage of 2.2 V” in Session We.2.5.2 of International conference, European Conference on Optical Communication 2003 (ECOC'03)
[Non-patent Document 8] 16th International Conference on Indium Phosphide Related Materials, ThA1-5. 2004
However, in Non-patent Document 1, the amplitude Vpp of the high-frequency driving power supply 61 is controlled in accordance with the wavelength of the input light, and therefore, it is necessary to prepare the high-frequency driving power supply 61 which makes the amplitude variable corresponding to the wavelengths of various input lights, but there exists the problem that use of such a high-frequency driving power supply 61 causes rise in cost of the optical transmitter, complication of the structure and huge apparatus construction.
In Non-patent Document 2, there is the problem that the quality deterioration of the optical output waveform occurs dependently on the wavelength of the input light.
Here, the explanation will be made more specifically with the example shown in FIG. 36. In FIG. 36, when the wavelength of the input light is long wavelength, the on level of light is smaller than the maximum optical output on the extinction curve. In order to obtain favorable optical modulation waveform in the Mach-Zehnder type optical modulator, it is generally necessary to modulate the maximum optical output intensity and the minimum optical output intensity on the extinction curve corresponding to the on level and the off level. This is because the gradient of the optical output intensity with respect to the input voltage V is zero at the maximum optical output intensity and the minimum optical output intensity of this extinction curve, and therefore, even if the voltage changes in the vicinity of this input voltage, it hardly has the influence on the optical output. The on level of the light in the case where the wavelength of the input light is long wavelength shown in FIG. 36 is not the maximum optical output on the extinction curve, and therefore, there is the fear that the on level of the light cannot be kept when the voltage change due to the influence of noise, bandwidth limitation and the like occurs to the input voltage V.
In the push-pull type Mach-Zehnder optical modulator as in Patent Document 1, the wavelength chirping amount can be controlled by adjusting the electrode length, and therefore, zero chirping can be obtained by properly designing the electrode lengths (specifically, by making the electrode lengths symmetrical). The same applies to the capacitively loaded type Mach-Zehnder modulator made of GaAs related materials, and zero chirping can be obtained by making the electrode lengths symmetrical. However, it has become apparent through the experiment by the inventor of the invention that even if the electrode lengths are made equal, and symmetrical between the two arms (optical waveguides) in the capacitively loaded type Mach-Zehnder modulator using InP related materials, zero chirping is not practically realized.
Here, the reason why zero chirping is not realized even if the electrode lengths are made equal, and symmetrical between the two arms (optical waveguides) is considered that the peculiar phenomenon to the InP related materials occurs, in which not only the refractive indexes of the optical waveguides but also the absorption indexes of the optical waveguides change at the same time by the voltage applied to the optical waveguides (arms) in the Mach-Zehnder modulator made of the InP related materials as described above. In the case of the capacitively loaded type electrode structure, the reason is further considered that the structures of the electrodes are asymmetrical between the two arms (optical waveguides). When the absorption indexes of the optical waveguides change with respect to the voltage, the way of influencing the wavelength chirping is complicated as compared with the case of the simple electrode structure as shown in FIGS. 37A and 37B because the structures of the electrodes are asymmetrical between the two arms (optical waveguides).
As described above, in the conventional capacitively loaded type Mach-Zehnder modulator made of the InP related materials, zero chirping is not obtained due to the optical absorption characteristic of the optical waveguides and asymmetrical electrode placement even when the electrode lengths are symmetrical. The relationship between asymmetry property of the electrode lengths and the wavelength chirping differs from the known relationship in an ordinary push-pull type Mach-Zehnder optical modulator, and is unclear. Therefore, in the capacitively loaded type Mach-Zehnder modulator made of InP related materials, the design of the optimal electrode lengths to obtain zero chirping is unclear, and as a result, zero chirping cannot be obtained by the adjustment of the electrode lengths as in the ordinary push-pull type Mach-Zehnder optical modulator. As a result, there arises the problem that the optical output waveform is lost and the quality deterioration of the optical output waveform occurs.